An arrhythmia is an abnormal heart beat pattern. One example of arrhythmia is bradycardia wherein the heart beats at an abnormally slow rate or wherein significant pauses occur between consecutive beats. Other examples of arrhythmia include tachyarrhythmias wherein the heart beats at an abnormally fast rate. With an atrial tachyarrhythmia, such as atrial tachycardia (AT), the atria of the heart beat abnormally fast. With a ventricular tachyarrhythmia, such as ventricular tachycardia (VT), the ventricles of the heart beat abnormally fast. Though often unpleasant for the patient, a tachycardia is typically not fatal. However, some tachycardias, particularly ventricular tachycardia, can trigger ventricular fibrillation (VF) wherein the heart beats chaotically such that there is little or no net flow of blood from the heart to the brain and other organs. VF, if not terminated, is fatal. Hence, it is highly desirable for implantable medical devices, such as pacemaker or ICDs (herein generally referred to as a pacer/ICD) to detect arrhythmias, particularly ventricular tachyarrhythmias, so that appropriate therapy can be automatically delivered by the device.
To detect arrhythmias, the pacer/ICD senses electrical cardiac signals within the heart of the patient using one or more implanted electrodes. The cardiac signals are sensed within the device by one or more sense amplifiers and then filtered by various filters configured so as to extract signals of interest, such as signals indicative of bradycardia or tachycardia or other arrhythmias. To this end, state-of-the-art pacer/ICDs are often provided with a wideband filter and two narrow bandwidth filters. The wideband filter eliminates low and high frequency noise but otherwise retains all features of the cardiac signals indicative of actual electrical events within the heart of the patient. That is, the wideband filter retains P-waves, R-waves and T-waves, whether occurring at normal heart rates, excessively low rates, or excessively high rates. The P-wave is the portion of an intracardiac electrogram (IEGM) signal that is representative of the electrical depolarization of the atria and is thus also representative of the physical contraction of the atria. The R-wave is the portion of the IEGM that is representative of the electrical depolarization of the ventricles and is thus also representative of the physical contraction of the ventricles. The T-wave is the portion of the IEGM that is representative of the electrical repolarization of the ventricles. (Note that the repolarization of the atria typically generates electrical signals that are too weak to be detected and hence atrial repolarization events are not typically detected by pacer/ICDs.)
Hence, within the wideband cardiac signals, the P-wave is typically followed by the R-wave, which is then followed by the T-wave. Note, however, that the wideband filter also retains signals associated with any chaotic or random beating of the chambers of the heart, particularly signals associated with VF, which may not be easily categorized as having discrete P-waves, R-waves or T-waves. Insofar as the R-wave is concerned, strictly speaking, the portion of the IEGM corresponding to the depolarization of the ventricles is referred to as the QRS complex, with the R-wave representing only a portion of that complex. However, the terms R-wave and QRS-complex are often used interchangeably in the literature when applied to the IEGM. Herein, the term “R-wave” is used to refer to the entire QRS-complex, unless otherwise noted.
FIG. 1 provides a stylized illustration of a cardiac signal 2 corresponding to a single heartbeat, particularly illustrating the R-wave 4 and the T-wave 6. In practice, the relative magnitudes of the various events can differ significantly. In some cases, the T-wave may be as large as or larger than the R-wave. Accordingly, it can be difficult to distinguish R-waves from T-waves from the wideband-filtered signals so as to obtain an accurate measure of the ventricular rate, and so it can be difficult to reliably detect either bradycardia or tachycardia from the wideband-filtered signals. Hence, the specialized narrowband filters are provided. A first narrowband filter, herein referred to as a bradycardia filter, is configured to filter the cardiac signal output from the sense amplifier so as to facilitate detection of only those features of the cardiac signals indicative of ventricular bradycardia. In particular, the bradycardia filter is designed to filter out substantially all portions of the cardiac signal not associated with non-VF R-waves. R-waves occurring during VF typically have a frequency too high to be detected by the bradycardia filter. Thus, the bradycardia filter provides an output signal that retains only relatively “slow” R-waves and eliminates substantially everything else (P-waves, T-waves, noise, etc.) from the raw cardiac signal sensed by the sense amplifiers. Bradycardia can thereby be conveniently detected by examining the filtered signal. If the rate at which R-waves appear in the filtered signal is below a lower rate threshold, or if no R-waves are present at all, then the patient is likely suffering an episode of bradycardia, and appropriate therapy can be delivered, such as demand-based pacing.
Advantageously, because all other features of the cardiac signals besides R-waves are filtered out by the bradycardia filter (i.e. T-waves, P-waves, noise, etc.), the sensitivity of the bradycardia filter can be set quite high so as to permit detection of even very low amplitude R-waves. The high sensitivity of the bradycardia filter thus substantially eliminates the risk of any possible undersensing of the R-waves (or at least any significant undersensing of non-VF R-waves.) Herein, “undersensing” refers to the failure to detect events of interest that are actually present within the raw cardiac signals. Meanwhile, the elimination of all other features of the cardiac signal by the filtering process (i.e. the elimination of P-waves, T-waves, etc.), means that there is little or no risk of “oversensing” of those other events when using the bradycardia filter. Herein, “oversensing” refers to the erroneous detection of an event not actually present in the raw cardiac signal, such as the detection of R-waves that are actually T-waves. Oversensing typically arises when one event is misidentified as another, as may occur, e.g., if a T-wave is improperly identified as an R-wave. As can be appreciated, T-wave oversensing is a significant concern since misidentification of T-waves as R-waves can result in significant miscalculation of the true heart rate within the patient, causing therapy to be delivered when not warranted or potentially causing therapy to be withheld even when needed.
A second narrowband filter, herein referred to as a tachycardia filter, is configured to filter the cardiac signal output from the sense amplifier so as to facilitate detection of only those features of the cardiac signals indicative of ventricular tachycardia. In particular, the tachycardia filter is designed to filter out substantially all portions of the cardiac signal not associated with relatively high rate R-waves, i.e. fast R-waves occurring at a rate consistent with VT of VF. Tachycardia can thereby be detected by examining the filtered signal. If the rate at which R-waves appear in the filtered signal is above a VT threshold, then the patient is likely suffering an episode of tachycardia, and appropriate therapy can be delivered, such as antitachycardia pacing (ATP) or shock. However, unlike the bradycardia filter, which fully eliminates T-waves, the tachycardia filter retains at least a portion of the T-wave. This is due to the fact that the frequencies associated with the fast R-waves of interest during VT are also associated with T-waves, and hence the filter cannot eliminate all T-waves while still retaining the R-waves. As such, the sensitivity of the tachycardia filter must be set so as to detect R-waves while eliminating T-waves. This is difficult, at best, since the relative magnitudes of the R-waves and T-waves may change significantly over time within the patient, perhaps due to the use of medications or due to physiological or anatomical changes in the heart brought on by medical conditions, such as cardiac ischemia, myocardial infarctions, congestive heart failure, etc. Moreover, as already noted, T-waves can sometimes have a magnitude that equals or exceeds that of the R-wave. Hence, T-wave oversensing is a significant problem within the tachycardia-filtered signals. If T-wave oversensing occurs, the ventricular rate cannot be accurately and reliably measured based solely on the output of the tachycardia filter (at least at the high rates associated with VT/VF), and hence problems arise in the detection of VT, VF or other forms of ventricular tachyarrhythmia. Failure to properly detect VT/VF when it is present can result in a failure to deliver appropriate therapy. False detection of VT/VF when it is not present can result in delivery of inappropriate therapy. As can be appreciated, both situations are of significant concern.
In view of the foregoing problems, it is highly desirable to provide improved techniques for reliably distinguishing different types of cardiac events within electrical cardiac signals and, in particular, to distinguish R-waves from T-waves so as to reduce or eliminate T-wave oversensing and thereby facilitate reliable detection of ventricular tachyarrhythmias. It is to this end that various aspects of the invention are generally directed.